GABA iontophoresis produced inhibitory effects on the firing rate of all somatosensory cortex neurons encountered. Other amino acids including glycine, G-alanine, and taurine had less potent inhibitory effects. All neurons were excited by glutamate or d,l-homocysteic acid. Penicillin iontophoresis produced an increase in spontaneous firing rate of most units studied. GABA inhibition is weakly antagonized by iontophoretic currents greater than 100 nA of penicillin. GAGA receptor antagonism is unlikely to be the primary mechanism by which penicillin produces acute epileptic foci.